The present invention relates to a process for preparing sol/gel particles which contain silicon dioxide and fillers and have a high breaking strength.
Silicon dioxide particles are widely used, for example, as catalysts, catalyst supports, adsorbents, drying agents or ion exchangers. For most of these intended uses, particles which have a high breaking strength are required.
Spherical silicon dioxide particles are usually prepared by the known sol/gel process. According to this process, described for example in French Patent No. FR 1,473,240, a silicon dioxide hydrosol is prepared by mixing an aqueous solution of an alkali metal silicate with an aqueous solution of an acid. The resulting hydrosol is converted into drop-shaped particles and the drops are then gelled in a water-immiscible liquid or in air. In a subsequent process step, a so-called base exchange is then carried out, in which the alkali metal content of the spherical silicon dioxide hydrogel particles is decreased in an aqueous medium to less than 1% by weight, based on the dry substance. The particles are then washed, dried and tempered.
It is known that the water-resistance and porosity of silicon dioxide particles can be modified by introducing small amounts of fillers into the particles. For some intended uses, for example, it may be desirable to have an aluminium oxide filler in the silicon dioxide particles. Furthermore, the production costs of the silicon dioxide particles can be reduced by mixing in an inexpensive filler.
During preparation of the silicon dioxide particles, a filler is added in a known manner by admixing the filler with the aqueous solution of the alkali metal silicate and/or with the aqueous solution of the acid, from which the hydrosol is obtained by mixing. In the past, substances such as kaolin, montmorillonite, bentonite, zeolites, amorphous alumosilicates, starch or wood flour, for example, have been used as fillers.
The processes of the prior art have the disadvantage that, because a small amount of filler is precipitated into the silicon dioxide particles, the breaking strength of the particles is greatly reduced. A high proportion of broken sol/gel particles during the preparation process increases the production costs of the particles. Sol/gel particles having insufficient breaking strength also are hardly suitable for the intended uses mentioned above.